One of the essential parameters of a multi-port amplifier is the isolation between the various output pathways which must be sufficient to guarantee acceptable isolation between the beams generated by the antenna. The isolation of a first output pathway in relation to a second output pathway different from the first is measured by the ratio of the maximum power of the amplified signal on the first output pathway of the multi-port amplifier to the residual power present on the second output pathway of the multi-port amplifier, this residual power being due to the imperfections of the system and also to the phenomenon of intermodulation.
A theoretical multi-port amplifier possesses infinite isolation between each of its output ports, however in practice, the Butler matrices and the amplification chains used within such an amplifier are not ideal and exhibit imperfections which may give rise to amplitude and phase errors which ultimately have an impact on the isolation of each of the output pathways in relation to the others. This phenomenon has notably all the more impact the higher the carrier frequency used. In view of these limitations, it is therefore necessary to produce a solution making it possible to correct the amplitude error and phase error and therefore to improve the overall isolation of the system.
U.S. Pat. No. 7,088,173 describes a scheme making it possible to correct the phase imperfections within a multi-port amplifier. The envisaged solution uses test signals injected at various points of the input network and measured at various points of the output network so as to determine the phase errors impacting the amplifiers. This solution makes it necessary to deactivate part of the amplifiers in order to perform the various measurements required. This scheme is therefore not suited to the simultaneous implementation of a communication.
U.S. Pat. No. 6,006,111 and Japanese patent JP2005269043 also describe methods making it possible to compensate for the imperfections of a distributed-amplification system on the basis of measurements performed on one or more output pathways but these schemes use only the measurement of the amplitude without taking the phase errors into account.
International patent application WO2008135753 describes a self-compensated multi-port amplifier solution comprising 8 input pathways and 8 output pathways. The principle consists in injecting a reference signal on the first input branch of the device and then measuring the imbalance between pathways, by virtue of 7 power detectors distributed in the output matrix. Several feedback loops then act on the attenuators and the phase shifters placed upstream of the amplifiers so as to minimize the voltage detected at each of the 7 points.
The scheme described in this patent application uses an iterative method for adjusting the amplitude and the phase upstream of the amplifiers so as to converge toward a zero power value on each point of the output matrix where a power detector is positioned. This method does not seem sufficiently efficacious to ensure sufficient isolation since the isolation matrix after compensation appears degraded in comparison with the isolation matrix before compensation. The result obtained is, in particular, not compatible with the proper operation of a telecommunications satellite mission.
Finally, American patent application US20080143562 may also be cited, which proposes a compensation scheme based on a priori knowledge of the signal transmitted by modeling its probability density. Such a scheme is too restrictive since it is not adaptable to all types of signal, in particular a signal modulated by the combination of several simple modulations.